Disc brakes are sometimes designed to be spring engaged and to be disengaged by an electromagnetic coil energized to attract an axially movable rotatable armature which acts to nullify the spring engagement force and release the brake. The electromagnetic coil, therefore, must be dimensioned to produce a sufficient electromagnetic force to overcome the force of the operator springs as well as residual magnetism forces. The force produced by the springs, in turn, controls the torque capacity of the brake. Thus, the size of the electromagnetic coil is a primary factor in the overall size of the brake unit and also in the cost of its manufacture.
Electromagnetic disc brake devices have been found to be particularly desirable in applications such as controlling computer memory discs. Computer discs have a high inertia yet must be stopped within a short predetermined period of time, for example, in case of a power failure to avoid damage to the pick-up head of the computer disc drive or alternately because of the requirements of the work cycle.
The disc brake construction is particularly advantageous in computer disc applications due to the flexibility of providing large axially directed flux paths, thereby providing a strong brake engaging force over a very short period of time. In addition, a brake utilizing a friction type engagement can readily incorporate a fail-safe feature in case of electrical failures.
The overall axial dimension of the braking device in computer applications, however, is critical, since it is desirable to design the brake with the minimum axial overall length. Hence, it is desirable to minimize the magnetic coil and the electromagnetic coil housing size. One such prior art design is shown in U.S. Pat. No. 4,280,073 to Miller issued July 21, 1981, owned by the assignee of the present application. This design incorporates a ball and pocket torque booster into the brake so as to increase the axial engagement force over that developed directly by the operation of the spring. This design reduces the spring force required for a given brake torque requirement, and hence, the required electromagnetic coil size. The torque booster action is not self-energizing so as to cease upon nullifying of the spring force.
In one embodiment of the aforementioned design, a rotationally fixed, axially movable armature plate is urged away from the coil housing by the spring operators and into engagement with a double-faced brake disc against a fixed braking plate, the brake disc being rotatably mounted to the hub to be braked. The brake disc is of two plate construction with a plurality of opposite pairs of ball pockets formed in adjacent opposite radial faces of each disc with a ball element disposed between each aligned pair. A washer spring urges the brake discs together. Whenever the coil is de-energized, the brake operator spring urges the friction surfaces into engagement with the armature and backing plate, with the reaction forces causing the ball elements to cam the armature into engagement with the corresponding pole face on the coil housing, thereby significantly increasing the engagement force generated by the brake operator springs. After energization of the coil, the spring operator force which is acting on the armature, is nullified, causing the release of the brake with the electromagnetic force required being less than the total axial force generated by the spring and torque booster combination.
The other embodiment of the aforementioned device is designed to be incorporated in an electric motor. Here, the ball elements are disposed in opposite pockets formed in the engaging faces of the electromagnetic coil housing and the armature respectively. The armature is provided with a friction disc spring which is urged into engagement with an annular brake surface provided on the motor fan assembly.
The aforementioned design has several drawbacks. For example, as the friction surfaces wear, the magnetic air gap between the armature and the ferromagnetic housing containing the coil becomes so great as to prevent the brake from disengaging. In addition, due to the increased force on the braking surface, the driving shaft connection of the rotatable member to the brake has a tendency to overstress the spline coupling between the hub and the brake discs. This increased stress produces cracks in the rotatable member resulting in premature failure of the braking device. Furthermore, the aforementioned design is limited to permit the use of frictional material on the rotatable member made from nonbrittle materials, such as asbestos, since the brittle materials, such as carbon graphite, would crack and fail prematurely presumably due to the shock loads applied thereto when the brake is applied. Finally, the aforementioned design is noisy in operation due to the tolerance in the spline connection between the hub and rotatable member.